1. Field of the Invention
The present invention relates to a method of preparing a bismuth oxide superconducting wire, and more particularly, it relates to an improvement for increasing the critical current density of a bismuth oxide superconducting wire.
2. Description of the Background Art
In recent years, superconductive materials of ceramics, i.e., oxide superconductive materials are watched with interest as those exhibiting higher critical temperatures.
In particular, a bismuth oxide superconductive material is expected for practical use due to a high critical temperature of about 110 K. It is known that a bismuth oxide superconductor has phases having critical temperatures of 110 K, 80 K and 10 K. It is also known that non-superconducting phases partially appear when a 110 K-phase superconductor is prepared in particular.
In relation to such a bismuth oxide superconductor, it is further known that a 110 K phase has a 2223 composition of Bixe2x80x94Srxe2x80x94Caxe2x80x94Cu or (Bi, Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cu and a 80 K phase has a 2212 composition of the same components.
On the other hand, an oxide superconducting wire is generally prepared by filling a metal pipe with oxide raw material powder, working the same into a wire through deformation processing such as extrusion, wire drawing or rolling, and thereafter heat treating the wire.
In order to work an oxide superconductor into an oxide superconducting wire and apply the same to a superconducting magnet, a wire for a device or a power cable, for example, it is important to attain a high critical current density, in addition to a high critical temperature. A current density exceeding at least 1000 A/cm2 must be attained in order to put an oxide superconducting wire into practice.
When a superconductor is worked into a long oxide superconducting wire, further, such a current density must be substantially uniformly attained over the longitudinal direction of the superconducting wire. While it is effective to employ a bismuth superconductor, particularly that containing Bi partially replaced by Pb, in order to increase the critical temperature, the current density of such a bismuth superconductor generally remains at about 100 to 200 A/cm2 at the most.
In practice, however, it is necessary to attain a current density of at least 10 times thereof, and such a high current density must be substantially uniformly attained over the longitudinal direction of the superconducting wire.
Accordingly, an object of the present invention is to provide a method of preparing a bismuth oxide superconducting wire, which exhibits a higher critical current density.
In order to solve the aforementioned technical problem, the inventive method comprises:
(1) A step of preparing powder of not more than 1 @m in mean particle diameter containing a mixture of superconducting phases mainly composed of 2212 phases of Bixe2x80x94Srxe2x80x94Caxe2x80x94Cu or (Bi, Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cu and non-superconducting phases obtained by calcining and pulverizing raw material powder of oxides, carbonate etc. at least once;
(2) A step of heat treating the powder at a high temperature;
(3) a step of coating the powder with a metal;
(4) a step of preparing a round wire by deformation-processing the as-formed composite of the powder and the metal coat;
(5) a step of deformation-processing the round wire thereby preparing a tape type or flat type wire;
(6) a step of heat treating the wire under conditions for allowing phase transformation of the 2212 phases contained in the superconducting phases to 2223 phases, and facilitating grain growth;
(7) a step of highly densifying the 2223 phases by deformation processing or pressurization; and
(8) a step of strongly bonding the 2223 phases with each other and finely dispersing the non-superconducting phases by a heat treatment.
In the step (2) which is carried out in advance of the step (3), the powder is preferably heat treated in the atmosphere, under decompression or in an inert gas, for the purpose of degassing.
In the step (5), the round wire is preferably worked into a tape type or flat type wire whose thickness is not more than 20% of the diameter of the round wire by single deformation processing.
The inventive method may further comprise a step of coating a plurality of the said round wires with a metal for obtaining a multifilamentary wire between the steps (4) and (5).
According to the present invention, it is possible to finely disperse non-superconducting phases in parts of 2223 phases in a superconductor portion of the as-obtained bismuth oxide superconducting wire. Thus, current paths are not hindered by the non-superconducting phases but connected with each other, whereby a bismuth oxide superconducting wire exhibiting a high critical current density is obtained. Since superconducting phases are formed with no hindrance by the non-superconducting phases, it is possible to obtain a bismuth oxide superconducting wire which has a substantially uniform critical current density over the longitudinal direction of the wire.
According to the present invention, therefore, obtained is a bismuth oxide superconducting wire having a high critical current density substantially uniformly over the longitudinal direction, whereby such a superconducting wire can be applied to a cable or a magnet with no problem.
According to the present invention, the aforementioned high critical current density is attained particularly since the powder is prepared to be not more than 1 xcexcm in mean particle diameter in the step (1), the composite is subjected to deformation processing (or pressurization) a plurality of times through the steps (4), (5) and (7) after the step (3), and the same is heat treated a plurality of times through the steps (6) and (8).
When the powder is heat treated at a high temperature in the step (2) which is carried out in advance of the step (3) for degassing, bonding properties at the grain boundaries of the 2223 phases are improved and the superconducting wire is prevented from swelling, whereby the superconductor portion contained in the superconducting wire is prevented from cracking etc. Thus, it is possible to remove a factor for reducing the critical current density in the superconducting wire.
When the round wire is strongly worked into a tape type or flat type wire whose thickness is not more than 20% of the diameter of the round wire in the step (5) through single deformation processing, a cross-directional flow of the metal-coated powder is further prompted. Since such a cross-directional flow is caused with further constraint by upper and lower rolls or the like as compared with a longitudinal flow, the powder is further densified to provide a higher critical current density.
When the inventive method further comprises a step of coating a plurality of the said round wires with a metal for obtaining a multifilamentary wire between the steps (4) and (5), the superconductor is sectionally distributed in a plurality of portions in the as-obtained bismuth oxide superconducting wire. Thus, a prescribed critical current density can be provided by a plurality of superconductor portions, and the thickness of each superconductor portion can be reduced. Therefore, it is possible to improve distortion resistance of the critical current density of the bismuth oxide superconducting wire.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.